Direct Sequence Spread Spectrum is a data transmission method that allows a number of wireless receivers to communicate on a common frequency spectrum by spreading respective transmission signals over the spectrum using unique code sequences for each transmission. Because multiple transmissions are being communicated on the common frequency spectrum at the same time, the respective communications thereon are indistinguishable from each other without knowing the corresponding code sequences. The code sequences, sometimes referred to a pseudo-noise codes, are employed to spread the desired signal on the transmission end and de-spread the desired signal on the reception end. At the transmission end, the desired waveform signal energy is spread over a bandwidth much greater than the signal information bandwidth. At the receiver end, the signal is de-spread using a synchronized replica of the pseudo-noise code.
In FIG. 1, a prior art spread spectrum receiver is shown. There, the baseband signals RXI and RXQ, which are composed of a series of symbols, are received at chip matched filters 10A and 10B. De-spreading occurs by correlation of the received symbols with locally generated pseudo-noise sequences PNI and PNQ from PN replicator 13. The correlations are obtained at the multipliers 12A and 12B and integrators 14A and 14B. The de-spread signals may be further processed via a signal acquisition block 15. The result from the signal acquisition block is the original desired signal.
The pseudo-noise codes employed in spreading and de-spreading take various forms, with different associated problems and advantages. The PN replicator 13 can also take various forms. The present invention is not limited to the kind or nature of the spreading code. One form of PN replicator employs a digitally matched filter that analyzes the incoming series of symbols to determine a match value (or “correlation”) identifying how well a subset series of symbols matches a known pseudo-noise code. The output of the digitally matched filter is thus a series of correlation values associated with different sets of received symbol strings.
DSSS transmission signals can be spread via the pseudo-noise code to signal levels that may fall below baseline noise levels. Very low signal to noise ratios are common and can result in poor signal acquisition. The present invention involves a method of establishing more robust signal acquisition at very low signal to noise ratios for DSSS signals. This more robust acquisition is achieved on a complex signal by four cross-coupled digitally matched filters operating simultaneously on the in-phase and quadrature components as well as the current and previous symbols.